Your Problem: You want to get that Blue Scholars song or the Les Miserables soundtrack from iTunes on your computer onto your iPhone, but you absolutely do NOT want to erase/delete all the music on your iPhone that comes with “syncing” your music to the iTunes library.
Short Answer: 1. Create a playlist (or use an old one) of the songs you want to add.
2. Under “Devices”, your iPhone should be displayed, showing that your iPhone is connected to your computer.
3. Under your iPhone, there is a “Music” tab for you to click on. Click on that.
4. At the bottom of iTunes there should be an option called “Autofill from” as you can see in the picture below. From “Autofill from”, you should be able to select that playlist of the music you added. In the picture below, the playlist I’ve selected is “New.” On the extreme right, click “Autofill.”
I give credit to the solution outlined here.
Disclaimer: My version of iTunes is 10.6.3 (25). I’m too lazy to check if it’s the absolute latest version of iTunes, but considering that my Macbook Pro is a 2012, it’s probably one of the more recent versions of iTunes. I don’t know any of the history of Autofill on iTunes, and I’m not in the curious mood right now. I am not sure how it works, but hey I’ve got my music.
Long Answer/My Story: For as long as I’ve had a 2nd Mac and my iPhone, I couldn’t add songs onto my iPhone. I searched far and wide for solutions, including Senuti, which only lets you add music from the iPhone onto the computer, without using iTunes.
I wasn’t able to add new music to my iPhone for quite a while.
Then voila, a few days ago, I ran into the solution in the link above.
I wrote this just as an affirmation and clearer set of instructions than described in the link.